The present invention pertains generally to the field of light projection systems, such as those used in movie theaters or other entertainment venues. More particularly, the present invention is directed to an apparatus for projecting a light, such as a laser light, onto a movie screen or other surface. The arrangement of lenses in the apparatus produces a beam that has a low energy density over most of the beam's distance, thereby reducing the risk of harm to a person sitting in the theater or venue.
In recent years, laser particle effect systems have been developed that enable a laser light to be used to simulate the appearance of certain effects that are difficult to reproduce with conventional video techniques, or that otherwise lend themselves to reproduction with lasers. Examples of such effects include visual effects like geometric patterns or writing on a surface, and abstract effects like glowing trails, magic spells, sparks, fizzing bubbles, pixie dust or the like.
The effects are created by rapidly moving a laser beam to create a desired pattern, or to create discrete dots of laser light, on a surface or projection screen or on multiple surfaces. The laser is rapidly cycled or interrupted to simulate a plurality of individual particles on the target surface. A plurality of different colored lasers may also be used in coordinated fashion to create multi-colored effects.
The laser particle effect may be overlaid onto, or otherwise coordinated with, a video that is projected onto the movie screen or other surface. To give the particle effect a three-dimensional characteristic, a mesh or “scrim” may be suspended in the path of the projection, onto which the particle effect is being projected. The mesh or scrim is normally not visible to the viewing patrons sitting in the audience, but will reflect the light that is being projected onto the mesh. A three dimensional effect can also be generated by placing other physical objects in front of or behind the plane of the primary screen, and shining the laser light onto those physical objects.
As an example, a movie theater will normally have a projector for projecting a film or video onto a projection screen. The projector is typically located in a projection room at the back of the theater, behind and above the seating area where the movie theater's customers sit while watching a movie. The projection screen is located at the other end of the theater, in front of the audience, with the audience sitting in chairs between the projection room and the projection screen.
A laser particle effect could be used in conjunction with advertisements shown before the main feature film. For example, an advertisement for a cola drink might include video showing the cola being poured into a glass, while a laser-based particle effect is superimposed over the video image that appears on the movie screen, to simulate fizzy bubbles coming off of the soda as it is poured into the glass. Particle effects can also be coordinated with and overlaid onto the video for a feature film, as part of the overall movie production.
Particle effect systems typically use a coherent light source, such as a laser light, to provide the particle effect. The advantage of using a coherent light source, such as a laser, is that the particle effect is made up of small, bright laser dots shining onto the viewing surface, making the effect brighter, more distinct and more vivid than if a non-coherent light source had been used.
However, there is a distinct disadvantage to using coherent light, such as laser light, to generate a particle effect in a crowded venue, such as a movie theater. Laser light typically has a high energy density. In crowded venues, there is always a risk that a person might position their body directly in the path of the laser beam, which can cause damage to the person's skin. Or, they might turn around and inadvertently look directly at the laser light, which can damage their eyes.
Because even relatively small amounts of laser light can lead to permanent eye injuries, the usage of lasers is typically subject to government regulations. To control the risk of injury, various specifications, for example ANSI Z136 in the US and IEC 60825 internationally, define “classes” of lasers, depending on their power and wavelength.
The maximum permissible exposure (MPE) is the highest power or energy density (in W/cm2 or J/cm2) of a light source that is considered safe. The MPE is measured at the cornea of the human eye or at the skin, for a given wavelength and exposure time. A calculation of the MPE for ocular exposure takes into account the various ways light can act upon the eye. In addition to the wavelength and exposure time, the MPE takes into account the spatial distribution of the light (from a laser or otherwise). Collimated laser beams of visible and near-infrared light are especially dangerous at relatively low powers because the lens focuses the light onto a tiny spot on the retina. Light sources with a smaller degree of spatial coherence than a well-collimated laser beam, such as high-power LEDs, lead to a distribution of the light over a larger area on the retina. For such sources, the MPE is higher than for collimated laser beams. In the MPE calculation, the worst-case scenario is assumed, in which the eye lens focuses the light into the smallest possible spot size on the retina for the particular wavelength and the pupil is fully open. Although the MPE is specified as power or energy per unit surface, it is based on the power or energy that can pass through a fully open pupil (0.39 cm2) for visible and near-infrared wavelengths. This is relevant for laser beams that have a cross-section smaller than 0.39 cm2. The IEC-60825-1 and ANSI Z136.1 standards include methods of calculating MPEs.
A Class 1 laser is safe under all conditions of normal use. This means the maximum permissible exposure (MPE) cannot be exceeded when viewing a laser with the naked eye or with the aid of typical magnifying optics (e.g. telescope or microscope). To verify compliance, the standard specifies the aperture and distance corresponding to the naked eye, a typical telescope viewing a collimated beam, and a typical microscope viewing a divergent beam. It is important to realize that certain lasers classified as Class 1 may still pose a hazard when viewed with a telescope or microscope of sufficiently large aperture. For example, a high-power laser with a very large collimated beam or very highly divergent beam may be classified as Class 1 if the power that passes through the apertures defined in the standard is less than the MPE for Class 1; however, an unsafe power level may be collected by a magnifying optic with larger aperture.
A Class 1M laser is safe for all conditions of use except when passed through magnifying optics such as microscopes and telescopes. Class 1M lasers produce large-diameter beams, or beams that are divergent. The MPE for a Class 1M laser cannot normally be exceeded unless focusing or imaging optics are used to narrow the beam. If the beam is refocused, the hazard of Class 1M lasers may be increased and the product class may be changed. A laser can be classified as Class 1M if the power that can pass through the pupil of the naked eye is less than the MPE for Class 1, but the power that can be collected into the eye by typical magnifying optics (as defined in the standard) is higher than the MPE for Class 1 and lower than the MPE for Class 3B.
A Class 2 laser is safe because the blink reflex will limit the exposure to no more than 0.25 seconds. It only applies to visible-light lasers (400-700 nm). Class-2 lasers are limited to 1 mW continuous wave, or more if the emission time is less than 0.25 seconds or if the light is not spatially coherent. Intentional suppression of the blink reflex could lead to eye injury. Many laser pointers and measuring instruments are class 2.
A Class 2M laser is safe because of the blink reflex if not viewed through optical instruments. As with class 1M, this applies to laser beams with a large diameter or large divergence, for which the amount of light passing through the pupil cannot exceed the limits for class 2.
A Class 3R laser is considered safe if handled carefully, with restricted beam viewing. With a class 3R laser, the MPE can be exceeded, but with a low risk of injury. Visible continuous lasers in Class 3R are limited to 5 mW. For other wavelengths and for pulsed lasers, other limits apply.
A Class 3B laser is hazardous if the eye is exposed directly, but diffuse reflections such as those from paper or other matte surfaces are not harmful. The MPE for continuous lasers in the wavelength range from 315 nm to far infrared is 0.5 W. For pulsed lasers between 400 and 700 nm, the limit is 30 mJ. Other limits apply to other wavelengths and to ultrashort pulsed lasers. Protective eyewear is typically required where direct viewing of a class 3B laser beam may occur. Class-3B lasers must be equipped with a key switch and a safety interlock. Class 3B lasers are used inside CD and DVD writers, although the writer unit itself is class 1 because the laser light cannot leave the unit.
Class 4 is the highest and most dangerous class of laser, including all lasers that exceed the Class 3B MPE. By definition, a class 4 laser can burn the skin, or cause devastating and permanent eye damage as a result of direct, diffuse or indirect beam viewing.
Most laser-based particle effect systems use Class 3B lasers, which normally require protective eyewear where direct viewing of a class 3B laser beam may occur. Consequently, current laser-based particle effect systems normally depend on placing the lasers in location that will negate the possibility of a person directly viewing the laser. This usually involves mounting the laser or lasers onto the ceiling in a theater or other entertainment venue, and shining the laser downward onto the movie screen or wall at a very steep angle. This precludes the use of a laser-based particle effect system that is located in the projection room at a back of theater. This can also make it more difficult to produce a video or film with coordinated, pre-planned, associated laser light effects, particularly where 3D effects are involved, because the producer of the coordinated media cannot know in advance that the video and particle effects will be projected from the same location.
In view of the foregoing disadvantages of existing particle effect systems, there is a need for an improved light projection apparatus that is capable of being positioned alongside, or incorporated into, a video projector. In a movie theater, the light projection apparatus could be positioned inside the projection room at the back of the theater. Because the light from the light projection apparatus (typically a laser light) would be shined over the audience sitting in the theater, there is a need for such an improved light projection system that projects a light with an energy density that is low enough to comply with the above-described regulations, to avoid potential harm to patrons in the audience. At the same time, there is a need for such an improved particle effect apparatus that will provide bright, distinct and vivid laser dots shining onto the movie screen or other viewing surface.